Terrigenous Clastic Composition and Its Tectonic Feature in the Mesozoic of the Ordos Basin
CHEN Gang (Department of Geology,Northwest University, Xi'an 710069)
The relationship between clastic composition and tectonic setting has provided a useful method to recognize the link of sedimentology and tectonic type.Based on the systematic analysis of regional sedimentary-tectonic character and clastic composition to determine the basin tectonic type of the Mesozoic in the Ordos Basin,it could be concluded as follows: First,there was a sinistral sheared stress field in mechanical property of the North China craton during the Mesozoic.There formed two major tectonic elements under the deformation of the stress field.One is the large uplifted zone in the east,and the other is the Ordos Basin in the west.Simultaneously,the west edge thrust belt of the basin was formed.The main structure of the Mesozoic in the Ordos Basin was presented as“deep depression in the west-slope and forebulge in the east”.Also,there were a large amount of conglomerate-clastic rocks of the Upper Triassic, the Upper Jurassic,and the Lower Cretaceous in the deep depression.These sedimentary tectonic charactes showed that the Mesozoic of the Ordos Basin is most likely to be a quasi-foreland basin. Meanwhile,the subsidence center of the Mesozoic in the Ordos Basin could not coincide with the sedimentary center,These two centers were not only changable but also located at different positions.From the late Triassic to the Early Cretaceous,the subsidence center of the west depression migrated along the thrust orientation,and the sedimentary center in the middle part of the basin circled anti-clockwisely.Obviously,It was the result of the west edge thrust belt of the basin and the sinistral sheared stress deformation of the North China craton during the Mesozoic. Futhermore,most of the Mesozoic samples fall in the Recycled Orogen Provenances in the QFL and QFLT triangular diagram delinested by Dickinson(1979),It is suggested that the clastic resources of the fills in the Mesozoic be like that of the foreland basin.However,the most chemical component ratios of the Mesozoic samples are not of the typical foreland basin character,but similar to the quasi-foreland basin.For example,Fe 2O 3+MgO=4%～6%,TiO 2=0.3%～0.5%,K 2O/Na 2O=0.8%～1.7%,Fe 2O 3/K 2O=1.1%～ 1.9% and SiO 2/Al 2O 3=5.5%～8.9%. Therefore,the Mesozoic Ordos Basin was not a typiocal foreland basin,but a quasi-foreland one .It was the most result of the west edge therust belt of the basin during the Meszozic, especially during the late Triassic,the late Jurassic and the Early Cretaceous.
【CateGory Index】： P618.130.2